1. Field of the Invention
The invention concerns a connection device with no electrical contact for making and breaking at will a connection conveying electrical power and/or an electrical signal from a first part of a machine to a second part.
2. Description of the Prior Art
Connection devices conventionally comprise electrical contacts and are subject to faults caused either by pollution, oxidation or wear or by impact during assembly or disassembly of the machine. French patent application no 2 655 753 describes a connection device with electrical contacts comprising two parts which plug into each other and a manually operated device for retracting one of these two parts to prevent impact between the two parts of the connection device during assembly or disassembly of the machine. This known device can thus prevent damage to the connection device during assembly or disassembly of the various parts of a machine but like all connection devices with electrical contacts it is subject to pollution, oxidation and wear of the contact members.
These phenomena are all the more marked in the case of connecting two parts of a machine which rotate relative to each other. In such cases it is known to use a connection device comprising at least one conductive ring and at least one conductive brush rubbing on this ring, the number of rings depending on the number of circuits to be made. A device of this kind wears particularly quickly if the relative movement is frequent or continuous. Also, assembling and disassembling a machine comprising a connection device of this kind is particularly difficult: there is always the risk of damaging the brushes or the rings.
It is known from U.S. Pat. No. 4,296,273 to replace rings and brushes connecting the fixed part of a magnetic tape recorder to a rotary drum carrying the record and playback heads and an electronic circuit with two rotary transformers, one conveying the electrical power to feed the electronic circuit in the drum and the other conveying either a video signal to be recorded on magnetic tape or a video signal read from magnetic tape. This American patent discloses an improvement to this device whereby a single rotary transformer may be used, i.e. a more compact system.
This device exploits one feature of video signals: the major part of each picture line period represents picture elements (pixels) and the minor part is used merely for synchronization and for the return movement (flyback) of the cathode ray tube scanning beam. The device transmits electrical power only during periods in which the video signal does not represent pixels. The device transmits the video signal during the remainder of each line period. The single transformer is thus used alternately to transmit power and to transmit a signal. This device is not subject to the wear characteristic of brush and ring devices and is less bulky than a connection device using two rotary transformers, but it nevertheless has the same two drawbacks: it makes assembly and disassembly of the drum difficult and it is too specifically adapted to the transmission of video signals. It cannot be used if it is necessary to transmit a signal to or from the drum continuously, as transmission of the signal must be interrupted periodically to transmit electrical power.
These two drawbacks arise in connection with establishing a connection between two parts of a mail franking machine. This type of machine usually comprises a fixed part or base incorporating an electrical power supply and a fixed but removable part or head which is fixed to the base during normal operation of the machine. The head may incorporate a rotary print drum which rotates relative to it. A franking machine comprises electronic circuits divided into several parts which must be interconnected firstly to receive electrical power from the power supply in the base and secondly to exchange digital signals at a rate of a few hundred kilobits per second. A connection device is specifically required to make or break at will a connection transmitting electrical power and electrical signals between the base and the head of the machine. The head includes electronic circuits for totalizing franking amounts. It is removable to enable the postal authority to totalize franking operations carried out on the basis of data stored in non-volatile electronic memory. The head must therefore be removed from and refitted to the base at frequent intervals. It is relatively heavy. If the head is connected to the base by a conventional connector the latter may be damaged by impact between its two parts. The device described in French patent application no 2 655 753 solves this problem by enabling retraction of one of the two parts of a conventional connector during the trial and error process of fitting the head to the base. However, it does not solve the problems of pollution, wear and oxidation of the metal contacts.
Consideration has been given to accommodating some of the electronic circuits inside the rotary print drum. A connection device is then required to connect the electronic circuits inside the drum to the electronic circuits in the remainder of the head. Although it is rarely necessary to demount the drum from the head it must be possible to do this without impediment by the connection device. Once again, assembly and disassembly must be simple and easy.
An object of the invention is to propose a connection device providing a secure connection, in particular through avoiding all problems of damage to metal contact members during assembly and disassembly, and which enables a connection to be made or broken at will. Another object of the invention is to propose a device which is able to convey electrical power and a signal simultaneously in one direction and optionally a second signal in the opposite direction.
The connection devices in franking machines may be misused in an attempt to sabotage the electronic circuits which totalize franking operations carried out, by applying high voltages to them. A further object of the invention is therefore to propose a connection device which is particularly resistant to any such sabotage attempts.